The steam volume fraction in the water coolant of a BWR increases with height in the reactor core due to the generation of steam along the length of the fuel rods in the boiling process. Thus, at the bottom of a BWR fuel assembly, there is essentially 100 percent unvoided water surrounding the fuel rods, but the percentage of unvoided water decreases up to the top of the fuel assembly. Near the bottom of the assembly, sufficient water exists to provide effective neutron moderation. However, due to the presence of steam in the upper part of the assembly, the assembly is under-moderated. The addition of water rods or water channels to the fuel assembly provides the water in the upper portion of the assembly that is needed to improve neutron moderation and hence uranium utilization efficiency.
BWR fuel designs typically have a varying number of fuel rods and water rods (or channels) in square arrays, such as 7.times.7, 8.times.8, 9.times.9, 10.times.10, or 11.times.11. The fuel rods and water rods (or channels) in the arrays are held in place by spacer grids distributed along the length and by tie plates at each end of the assembly. The fuel rods and water rods extend through the full length of the fuel assembly, i.e. the cross section of the active fuel zone is uniform over the full length of the assembly.
Early BWR fuel designs usually employed one or two water rods near the center of the fuel assembly which displaced one or two fuel rods. The purpose of the water rods was to put more water into the center of the assembly to improve neutron moderation and uranium utilization in the surrounding fuel rods.
More recent designs have employed an even greater number of water rods or large water channels. For example, Advanced Nuclear Fuels Corporation of Richland, Wash., U.S.A. has fabricated a 9.times.9-5 design which includes 76 fuel rods and 5 water rods. Also, lead assemblies of a 9.times.9-IX design have been sold which includes 72 fuel rods and an internal water channel that displaces 9 fuel rods.
While these designs improve the water moderation and uranium utilization in the fuel assembly, replacement of the fuel rods with water rods requires the linear heat generation rate (LHGR) to increase in the remaining fuel rods in order to produce equivalent energy in the entire assembly. Thus, the peak LHGRs increases as fuel rods are replaced with water rods. This can increase the probability of fuel rod failures during reactor power maneuvers due to pellet clad interaction (PCI). Also, the higher LHGRs reduce the margin to reactor operating or Technical Specification limits.